Some chemical substances are known to show a promoting effect on the growth of plants, when the plants are treated with such a substance. For example, aminolevulinic acid shows a promoting effect on the growth of plants, when the compound is applied to the plants.    [Non-patent literature 1] “Biosynthesis, biotechnological production and applications of 5-aminolevulinic acid” K. Sasaki et al., (2002) Applied Microbial Biotechnology 58: pp. 23-29